The present invention generally relates to the control of fluid flow about solid surfaces and, more particularly, to a synthetic fluid actuator embedded in a solid surface downstream from an obstruction on the solid surface such as to emit a synthetic jet stream out of the surface and modify the characteristics of fluid flowing over and about the surface.
The ability to manipulate and control the evolution of shear flows has tremendous potential for influencing system performance in diverse technological applications, including: lift and drag of aerodynamic surfaces, flow reattachment to wings, and aircraft stall management. That these flows are dominated by the dynamics of a hierarchy of vortical structures, evolving as a result of inherent hydrodynamic instabilities (e.g., Ho and Huerre, 1984), suggests control strategies based on manipulation of these instabilities by the introduction of small disturbances at the flow boundary. A given shear flow is typically extremely receptive to disturbances within a limited frequency band and, as a result, these disturbances are rapidly amplified and can lead to substantial modification of the base flow and the performance of the system in which it is employed.
There is no question, that suitable actuators having fast dynamic response and relatively low power consumption are the foundation of any scheme for the manipulation and control of shear flows. Most frequently, actuators have had mechanically moving parts which come in direct contact with the flow [e.g., vibrating ribbons (Schubauer and Skramstad J. Aero Sci. 14 1947), movable flaps (Oster and Wygnanski, 1982), or electromagnetic elements (Betzig AIAA, 1981)]. This class of direct-contact actuators also includes piezoelectric actuators, the effectiveness of which has been demonstrated in flat plate boundary layers (Wehrmann 1967, and Jacobson and Reynolds Stan. U. TF-64 1995), wakes (Wehrmann Phys. Fl. 8 1965, 1967, and Berger Phys. Fl. S191 1967), and jets (Wiltse and Glezer 1993). Actuation can also be effected indirectly (and, in principle, remotely) either through pressure fluctuations [e.g., acoustic excitation (Crow and Champagne JFM 48 1971)] or body forces [e.g., heating (Liepmann et al. 1982, Corke and Mangano JFM 209 1989, Nygaard and Glezer 1991), or electromagnetically (Brown and Nosenchuck, AIAA 1995)].
Flow control strategies that are accomplished without direct contact between the actuator and the embedding flow are extremely attractive because the actuators can be conformally and nonintrusively mounted on or below the flow boundary (and thus can be better protected than conventional mechanical actuators). However, unless these actuators can be placed near points of receptivity within the flow, their effectiveness degrades substantially with decreasing power input. This shortcoming can be overcome by using fluidic actuators where control is effected intrusively using flow injection (jets) or suction at the boundary. Although these actuators are inherently intrusive, they share most of the attributes of indirect actuators in that they can be placed within the flow boundary and require only an orifice to communicate with the external flow. Fluidic actuators that perform a variety of xe2x80x9canalogxe2x80x9d (e.g., proportional fluidic amplifier) and xe2x80x9cdigitalxe2x80x9d (e.g., flip-flop) throttling and control functions without moving mechanical parts by using control jets to affect a primary jet within an enclosed cavity have been studied since the late 1950""s (Joyce, HDL-SR 1983). Some of these concepts have also been used in open flow systems. Viets (AIAA J. 13 1975) induced spontaneous oscillations in a free rectangular jet by exploiting the concept of a flip-flop actuator and more recently, Raman and Cornelius (AIAA J. 33 1995) used two such jets to impose time harmonic oscillations in a larger jet by direct impingement.
More recently, a number of workers have recognized the potential for MEMS (micro eclectro mechanical systems) actuators in flow control applications for large scale systems and have exploited these devices in a variety of configurations. One of a number of examples of work in this area is that of Ho and his co-investigators (e.g., Liu, Tsao, Tai, and Ho, 1994) who have used MEMS versions of xe2x80x98flapsxe2x80x99 to effect flow control. These investigators have opted to modify the distribution of streamwise vorticity on a delta wing and thus the aerodynamic rolling moment about the longitudinal axis of the aircraft.
It was discovered at least as early as 1950 that if one uses a chamber bounded on one end by an acoustic wave generating device and bounded on the other end by a rigid wall with a small orifice, that when acoustic waves are emitted at high enough frequency and amplitude from the generator, a jet of air that emanates from the orifice outward from the chamber can be produced. See, for example, Ingard and Labate, Acoustic Circulation Effects and the Nonlinear Impedance of Orifices, The Journal of the Acoustical Society of America, March, 1950. The jet is comprised of a train of vortical air puffs that are formed at the orifice at the generator""s frequency.
The concern of scientists at that time was primarily with the relationship between the impedance of the orifice and the eddies (vortical puffs, or vortex rings) created at the orifice. There was no suggestion to combine or operate the apparatus with another fluid stream in order to modify the flow of that stream (e.g., its direction). Furthermore, there was no suggestion that following the ejection of each vortical puff, a momentary air stream of xe2x80x9cmake upxe2x80x9d air of equal mass is drawn back into the chamber and that, as a result, the jet is effectively synthesized from the air outside of the chamber and the net mass flux out of the chamber is zero.
Even though a crude synthetic jet was known to exist, applications to common problems associated with other fluid flows or with lack of fluid flow in bounded volumes were not even imagined, much less suggested. Evidence of this is the persistence of certain problems in various fields which have yet to be solved effectively.
The capability to alter the aerodynamic performance of a given airframe by altering its shape (e.g., the xe2x80x9ccamberxe2x80x9d of an airfoil) during various phases of the flight can lead to significant extension of the airframe""s operating envelope. Geometric modification of lifting surfaces has so far been accomplished by using mechanical flaps and slats. However, because of the complex control system required, such devices are expensive to manufacture, install and maintain. Furthermore, flap systems not only increase the weight of the airframe, but also require considerable interior storage space that could be used for cargo, and additional ancillary hardware (e.g., hydraulic pumps, piping, etc.). In some applications, the weight penalty imposed by the flaps may more than offset their usefulness.
Much of the recent work on flow control techniques with the objective of extending the post stall flight envelope of various airfoil configurations has focused on the manipulation of flow separation at moderate and large angles of attack either at the leading edge or over flaps (e.g., Seifert et al., Oscillatory Blowing: A Tool to Delay Boundary-Layer Separation, AIAA, 1993). This has been typically accomplished by exploiting the instability of the separating shear layer and its receptivity to time-periodic actuation (e.g., pulsed blowing) on the time scale of the flow about the airfoil, which results in a Coanda-like unsteady reattachment. Active control techniques that have achieved varying degrees of separation control by manipulation of the unstable separated free shear layer have included external and internal acoustic excitation (e.g., Ahuja and Burrin, Control of Flow Separation by Sound, AIAA, 1984, and Hsiao et al., Control of Wall-Separated Flow by Internal Acoustic Excitation, AIAA, 1990), vibrating ribbons or flaps (e.g., Neuburger and Wygnanski, The Use of a Vibrating Ribbon to Delay Separation on Two Dimensional Airfoils, TR-88-0004, 1987), and steady and unsteady blowing or bleed (e.g., Williams et al., The Mechanism of Flow Control on a Cylinder with the Unsteady Bleed Technique, AIAA, January, 1991, and Chang et al., Forcing Level Effects of Internal Acoustic Excitation on the Improvement of Airfoil Performance, Journal of Aircraft, 1992). In these experiments, the time-periodic actuation was typically applied at a dimensionless (reduced) frequency, F+xcx9cO(1) such that the actuation period scaled with the time of flight over the length of the reattached flow.
More recently, Smith et al. (Modification of Lifting Body Aerodynamics using Synthetic Jet Actuators, AIAA, January, 1998) and Amitay et al. (Flow Reattachment Dynamics over a Thick Airfoil Controlled by Synthetic Jet Actuators, AIAA, January, 1999 and Modification of the Aerodynamics Characteristics of an Unconventional Airfoil Using Synthetic Jet Actuators, AIAA Journal, June, 2001) demonstrated the suppression of separation over an unconventional airfoil at moderate Reynolds numbers (up to 106) that resulted in a dramatic increase in lift and a corresponding decrease in pressure drag. Actuation was effected using synthetic (zero mass flux) jet actuators, which were deliberately operated at frequencies that were typically an order of magnitude higher than the characteristic (shedding) frequency of the airfoil [i.e., F+xcx9cO(10) rather than F+xcx9cO(1)]. These authors argued that the interaction of high-frequency zero net mass flux jets with the cross flow leads to local modification of the apparent aerodynamic shape of the flow surface, and, as a result, to full or partial suppression of flow separation. Moreover, the recent experiments of Erk (Separation Control on a Post-Stall Airfoil Using Acoustically Generated Perturbations, Ph.D. Dissertation, 1997) demonstrated the suppression of separation on an FX61-184 airfoil at Reynolds numbers up to 3xc3x97106 using synthetic jet actuation at frequencies up to F+xcx9cO(100).
The interaction between a cross flow over a 2-D circular cylinder and surface-mounted synthetic jet actuators was recently investigated in detail by Honohan et al. (Aerodynamic Control Using Synthetic Jets, AIAA, June, 2000). These authors showed that when the jets are operated on a time scale that is well below the characteristic time scale of the base flow, their interaction with the cross flow leads to the formation of distinct quasi-steady flow regions near the surface and displacement of the local streamlines induces an apparent or virtual change in the shape of the surface and in the local pressure gradient. The acceleration of the cross flow around the interaction domain is accompanied by substantial alterations of the streamwise pressure gradient both locally and globally. As a result the surface boundary layer downstream of the interaction domain becomes thinner allowing the flow to overcome stronger adverse pressure gradients and therefore delaying (or altogether suppressing) flow separation.
The concept of modifying the apparent aerodynamic shape of aero-surfaces in order to prescribe the streamwise pressure distribution and therefore to influence its aerodynamic performance is not new and was addressed in U.S. Pat. No. 5,758,853, to Glezer, et al., entitled xe2x80x9cSynthetic Jet Actuators and Applications Thereof.xe2x80x9d This patent disclosed the use of synthetic jet actuators for the modification of fluid flow, generally. Additionally, U.S. Pat. No. 5,758,853 also discussed the use of synthetic jet actuators embedded into solid surfaces in order to alter the apparent aerodynamic shape of these surfaces.
Recent studies have suggested that the use of synthetic jet actuators in aerodynamic surfaces at relatively low levels of actuation [i.e., momentum coefficient (Cxcexc) of O(10xe2x88x923), where Cxcexc, is the momentum ratio between the jet and the free stream] and high enough operating frequencies is very effective at post stall angles of attack of the baseline airfoil, because the actuation leads to alteration of the streamwise pressure gradient and therefore to the suppression of separation. However, at low angles of attack in the absence of stall, the induced aerodynamic changes require higher actuation levels (or higher Cxcexc) and therefore an increase in energy usage, and possibly size of the synthetic jet actuator. While this energy cost may be acceptable for short periods of time (e.g., during maneuvering), it would be desirable to have a method and device for altering the aerodynamic characteristics with synthetic jet actuators at low actuation levels for prolonged actuation periods such as flight in cruise conditions.
Briefly described, the present invention involves the use of synthetic jet actuators positioned downstream from miniature surface-mounted passive obstructions for modification of fluid flow about various aerodynamic surfaces. Particularly, the present invention is concerned with a dramatically new approach to actually altering the apparent aerodynamic shape of various solid bodies at low angles of attack where the baseline flow is fully attached.
The devices capable of forming synthetic jets all have certain features common to the class of synthetic jets, which enable these devices to have novel applications for the modification of fluid flow about a solid body or surface. In particular, the present invention involves the modification of fluid flow about a body by altering the apparent aerodynamic shape of the body when the body is immersed in a fluid flow field. A brief description of the novel apparatus in process to which the present invention is directed follows:
For such an application, a synthetic jet actuator is preferably embedded in a solid body, or surface, downstream from a relatively small surface-mounted obstruction, with the jet orifice built into the body surface. The interaction of the fluid flow about the body with a synthetic jet stream produced by the actuator will change the overall fluid flow field around the solid body.
A unique feature of synthetic jet actuator arrays is that they can effectively modify wall-bounded shear flows by creating closed recirculating flow regimes near solid surfaces. In fact, the synthetic jet fluid actually penetrates the flow boundary layer to affect the overall flow field about the solid body. The interaction domain between a synthetic jet and the cross flow over a solid surface leads to a change in the apparent aerodynamic shape of the surface; hence they can be exploited for modification of aerodynamic performance measures such as lift or drag.
The preferred embodiment for the present invention is use of one or more synthetic jet actuators to modify the aerodynamic shape of a lifting surface in a flow field. Such a lifting surface will typically comprise a wing, the fuselage, a rotor blade, etc. In particular, a synthetic jet actuator embedded in a solid lifting surface either downstream (or upstream) from a miniature surface-mounted obstruction enables the formation and control of a permanent recirculation region near the jet orifice at both relatively low angles of attack and relatively low-level actuation. The obstruction pushes the fluid of the flow field away from a top surface of the lifting surface and generates a vortex downstream of the obstruction. The synthetic jet actuator entraps, controls and regulates this vortex in a region of recirculating flow. The recirculation region has the effect of modifying both the flow field and pressure distribution around the aerodynamic lifting surface, thereby modifying both lift and drag characteristics of the surface. The synthetic jet actuator can be embedded in the solid lifting surface upstream of the obstruction such that it is in fluid communication with a low pressure region that forms forward of the obstruction. In this position, the synthetic jet actuator can be used to form a permanent recirculation region near the jet orifice and similarly modify the flow field as previously discussed. Moreover, arrays can be placed both forward and aft of the obstruction and selectively energized as desired.
In particular, because the aerodynamic characteristics of an airfoil depend critically on its camber and thickness, these characteristics can be altered by synthetic jet actuators without the use of movable flaps. Similarly, placement of jet arrays and the creation of closed recirculating flow regions along both the upper (suction) and lower (pressure) surfaces of an airfoil can result in changes in its apparent thickness and camber. It should be noted that the obstruction can be designed to effect the desired local changes in flow direction with minimal drag penalty by either being attached to the surface or by being slightly displaced from the surface to allow for surface venting.
Other features and advantages will become apparent to one with skill in the art upon examination of the following drawings and detailed description. All such additional features and advantages are intended to be included herein within the scope of the present invention, as is defined by the claims.